Bone marrow derived professional antigen presenting cells (pAPC) are required to initiate anti viral cytotoxic T lymphocytes (CTL) responses. However, when a virus cannot infect the relevant pAPC, CTL responses are still generated because pAPC can acquire antigen from other infected cells and present it to CTL. This process is known as cross-presentation. The overall objective of this project is to define the mechanisms of MHC class I, antigen cross-presentation that occur in viral infections and result in successful CTL priming. For this purpose we will use unique in vitro and in vivo models of viral infection where antigen presentation occurs exclusively, through cross-presentation. In our first aim we will analyze which are the types of virus that are more successful, at inducing cross-presentation. We will also determine whether the most likely relevant pAPC dendritic cell, and macrophages, are equally efficient in cross-presentation and whether cross-presentation is accompanied by signs of dendritic cell or macrophage activation. In aim 2 we will determine in vitro the general characteristics of cross-presented antigens such as stability, solubility and sub-cellular localization. In addition we will, determine the relationship between cross-presentation and the mechanism of death whereby the infected cell dies, and the route followed by the antigen within the pAPC. In aim 3, the issues of proteins stability, subcellular localization and mechanisms of cell death will be analyzed in vivo. In addition, aim 3 will look at the role and mechanisms of T cell help in the induction of anti-viral CTL responses by cross-presentation. In summary, in this application we will address some questions that are fundamental to our understanding of CD8+ T lymphocyte immunity. The discovery of the mechanisms that regulate antigen presentation to CTL may provide a basis for the development of vaccines and immunotherapies for viral diseases and cancer and for the understanding, prevention and treatment of autoimmune diseases.